PULSE-TRACING. 163 



In the radial artery tracings, on the other hand, we see 

 that the up-stroke is single. In this case the percussion- 

 impulse although it causes the pulse to begin at the same 

 moment with that in the carotid, is not sufficiently strong 

 to jerk up the lever and produce an effect distinct from 

 that of the systolic wave which immediately follows it, and 

 which continues and completes the distension. In cases 

 of feeble arterial tension, however, the percussion-impulse 

 may be traced by the sphygmograph, not only in the 

 carotid pulse, but to a less extent in the radial also. 



In looking now at the down-stroke (fig. 44) in the 

 tracings, we see that it is interrupted by a well-marked 

 notch, or in other words, that the descent is interrupted 

 by a slight up-rising. In some cases of disease this 

 re-ascent is so considerable as to be perceptible to the 

 finger, and this double-beat has received the technical 

 name of " dicrotous " pulse. As a diseased condition this 

 has been long recognized, but it is only since the invention 

 of the sphygmograph that it has been found to belong in 

 a certain degree to the normal pulse also. 



Various theories have been framed to account for this 

 dicrotism of the normal pulse, but the most probable, and 

 that which is generally adopted, supposes it to be due to 

 the aortic valves, the sudden closure of which stops the 

 incipient regurgitation of blood into the ventricle, and 

 causes a momentary rebound throughout the arterial sys- 

 tem. As before remarked, when the tone of the artery is 

 good, the dicrotism is less marked ; and it is often replaced 

 by a series of very slight vibrations. 



The beginning of the down-stroke, the part, namely, 

 between the end of the up-stroke and the beginning of the 

 slight re-ascent, represents, probably, not as has been 

 supposed, some regurgitation into the ventricle before the 

 final closure of the semilunar valves, but simply the com- 

 mencing subsidence of the artery's tension, after the 

 passage of the systolic wave before referred to. 



